Protective relaying modem receiver

Abstract

A receiver is provided for utilization in a protective relaying modem to effectively distinguish between two mutually exclusive signal frequencies in a given frequency channel and which are respectively indicative of a "fault" or "no-fault" condition in a remote power transmission line. The received signals are coupled to a channel detector which utilizes a local oscillator and mixers to translate the received signal down in frequency so that simple low pass filters can isolate the desired frequency channel. The outputs of the low pass filters are operated on by a combination of multipliers and differentiators to generate an AM related output and a FM related output. The FM signal output of the channel detector is DC in nature and has an amplitude proportional to the deviation of the received signal frequency from the center frequency of the channel. This signal is used by separation circuitry to ascertain a fault or no-fault condition. The AM output is a DC signal that is proportional to the strength of the received signal and is analyzed to determine if the originally received signal was generated or unacceptably affected by electronic noise. If the noise content does not fall within the predeterminably set limits, any "fault" condition signal is automatically inhibited until the validity of such signal can be ascertained. The detection time for ascertaining whether a valid fault signal has been received can be adjusted to achieve a minimum yet effective detection time. The output of the receiver indicating a fault condition can be utilized to control various circuit breakers in the power transmission system.

I claim: 
     
       1. A receiver for receiving and distinguishing between first and second mutually exclusive signal frequencies transmitted in a defined frequency channel of center frequency f o  by remote transmitting means which comprises: an input means for receiving a signal that is at said first or said second signal frequency;   channel detector means turned to the frequency f o  coupled to said input means and generating first and second DC signals appearing at first and second channel detector outputs respectively, said first DC signal being proportional to the difference in frequency between the center frequency f o  and the frequency of the received signal, said first DC signal thereby having either a first or a second DC amplitude, and said second DC signal being proportional to the amplitude of the received signal;   separator means coupled to said first channel detector output for generating a first separator output signal when said first DC signal is at said first DC amplitude and a second separator output signal when said first DC signal is at said second DC amplitude;   comparator means coupled to said second channel detector output for generating an output inhibit signal when the amplitude of said second DC signal varies by a predetermined amount;   gating means coupled to said comparator output and responsive to said inhibit signal, said gating means for inhibiting said first separator output signal; and   timing means having an input coupled to said separator output, and being responsive only to said first separator output signal, said timing means for generating a control signal after said first separator output signal is present for a predetermined period of time at said timing input, said control signal being operable to control electrical functions external to the receiver.   
     
     
       2. A receiver as in claim 1 wherein said input means further comprises an automatic gain control amplifier providing a received signal of constant power to said channel detector means, said received signal having the form 2a sin ω 1  t, where 2a represents the amplitude, ω 1  represents the angular frequency of the received signal, and t represents the time variable. 
     
     
       3. A receiver as in claim 2 wherein said channel detector further comprises: a local oscillator generating the quadrature signals sin ω o  t; cos ω o  t;   first and second frequency translating means each coupled to said input means and to said local oscillator, said first frequency translating means producing the output signal a cos (ω 1  - ω o )t and said second frequency translating means producing the output signal a sin (ω 1  - ω o )t;   first derivative means responsive to the output signal of said first frequency translating means for producing an output signal proportional to the derivative thereof;   second derivative means responsive to the output signal of said second frequency translating means for producing an output signal proportional to the derivative thereof;   first multiplier means responsive to the output signal from said first derivative means and to the output signal from said second frequency translating means for producing an output signal of the form - a 2  (ω 1  - ω o ) sin 2  (ω 1  - ω o ) t;   second multiplier means responsive to the output signal from said second derivative means and to the output signal of said first frequency translating means for producing an output signal of the form a 2  (ω 1  - ω o )cos 2  (ω 1  - ω o )t; and   means for combining the output signals of said first and second multiplier means for producing an output signal of the form a 2  (ω 1  - ω o ).   
     
     
       4. A receiver as in claim 3 wherein said comparator means further comprises: a first amplitude comparator producing an output signal when the amplitude of said second DC signal exceeds a predetermined upper amplitude threshold; and   a second amplitude comparator producing an output signal when the amplitude of said second DC signal is below a predetermined lower amplitude threshold.   
     
     
       5. A receiver as in claim 4 wherein said gating means further comprises: an OR gate responsive to the output signals from said first and second amplitude comparators for generating said inhibit signal.   
     
     
       6. A receiver as in claim 5 further comprising: a noise detector responsive to said second DC signal for producing an output signal whose amplitude is proportional to the noise level of said received signal;   a third amplitude comparator responsive to the output signal of said noise detector for producing an output signal when said noise level exceeds a predetermined noise threshold, said output signal from said third comparator coupled to said OR gate.